With the rapid development of integrated circuit fabrication technology, more and more devices are incorporated on a single integrated circuit (IC) chip, and the size of each device on the IC chip and the spacing between the devices (i.e., feature size) continue to decrease. As feature sizes have become smaller, the demand for higher aspect ratios (i.e., a ratio between the depth and the width of a feature) has steadily increased. It is often difficult to deposit conductive materials into features (e.g., contact holes, via holes) with high aspect ratios to form contacts without generating certain undesirable defects (e.g., seams, voids).